CMOS technology is used for digital integrated circuits due to low power dissipation, high density of integration, and low cost of fabrication. CMOS technology is also used for analog integrated circuits.
Applications that use microelectronic components, such as telecommunications equipment, industrial control equipment, automotive electronics, etc., require more and more specialized integrated circuits. The continuing development in semiconductors has led to use of gate arrays and standard cells as a modern and inexpensive way to produce Application Specific Integrated Circuits (ASICs). An ASIC is an integrated circuit that can place on a single chip an entire system or a great part of it, performing not only digital, but also analog functions.
Gate arrays are used in ASIC design. A CMOS gate array can be described as a matrix of premanufactured (e.g., identical) cells that only requires the addition of the final metal and contact masks to define a new circuit function. Gate array technology can thus quickly respond to customer requirements in a low cost and efficient manner. Gate arrays can be implemented using a variety of circuit and process technologies including static CMOS and bipolar emitter coupled logic.
FIG. 1 shows a prior art static CMOS logic circuit 10. A problem with static CMOS logic circuits is that each input 12 and 14 must drive two gates, the gate of one NMOS transistor and the gate of a PMOS transistor. Input 12 drives gates 16 and 18, and input 14 drives gates 20 and 22. This results in a large area for static CMOS circuits and a large number of metal wiring levels must be utilized to allow interconnections.
Another problem with static CMOS logic circuits is that in the PMOS transistor the hole mobility is about three times lower than the mobility of electrons if the transistors have comparable sizes. Because of this, switching transients are very asymmetrical. The charge up transient of the capacitive load in a simple inverter takes far longer than the discharge transient. To attempt to compensate, the PMOS transistors are often fabricated with a large width or size to provide symmetrical switching. However, this increases the stray capacitive loads and results in an even larger area for the circuits, and very inefficient area utilization.